It sounds like the vet is thinking along the lines of inflammatory bowel disease. Has his/her eisinafil (sic) levels been tested? Also, if the vet is thinking along the lines of IBD, try a course of steroids first.....the exploratory surgery is the sure thing, but, the treatment is the same. Have you noticed loose stools?

I wonder about the second test for the thyroid...what is it? Can your vet feel an enlarged thyroid? I would think with such a weight loss, you would feel it. Food allergies? Still eating? Renal failure? All things to consider.

Good luck.